1. Field of the Invention
The present invention generally relates to an optical disk device. More specifically, the present invention relates to an optical disk device that records information to a recording medium.
2. Background Information
CD-R disks, CD-RW disks, DVD-R disks, DVD-RW disks, DVD+R disks, DVD+RW disks, DVD-RAM disks, DVD+RAM disks, and other such optical disk medium (e.g., recording medium or optical disk) have been used for some years now. CD-R/RW drives, DVD-R/RW drives, DVD+R/RW drives, DVD-RAM drives, DVD+RAM drives, and other such information recording devices used to record information to the optical disk medium are designed so that rotation of the optical disk medium is controlled, recording power of laser beam (e.g., light beam) emitted from a light source is controlled to a target value, and information is recorded by converging the laser beam emitted from the light source on the optical disk medium.
With the optical disk devices, an optimal laser power for recording to the optical disk medium (e.g., optimal recording power) varies according to conditions, such as recording characteristics of the optical disk medium, recording speed, and variance in performance between individual devices. Thus, processing for determining the optimal recording power has to be carried out prior to recording actual information (e.g., actual data) to the optical disk medium.
For example, when the information is recorded to a DVD-RW disk, test information is written while progressively varying the recording power in a test recording area (e.g., power calibration area; PCA) of the optical disk medium. Then, an optimal recording power is determined by detecting signal for the test information written in the test recording area. This determination of the optimal recording power is called optimum power control (OPC) in the case of CD-R disks. A specific range over which the recording power is progressively varied during the OPC is called an “OPC range.”
Subsequent recording operations are carried out by emitting the laser beam at the optimal power ascertained by the OPC processing. In recording (specifically, overwriting) of new information in a recording area where information has already been recorded, the laser beam is emitted at the optimal power directly without performing an independent erasure operation. This is generally referred to as a direct overwrite.
The recording characteristics for the DVD-RW disks and other such optical disk medium will vary with types of optical disk medium being used. Therefore, the recording power needed to record information will also vary, which means that with the optical disk devices, a median power (Pdef) of the recording power is often stored separately in executing the OPC for each type of optical disk medium. Also, the recording characteristics vary greatly among individual optical disk medium, or depending on a manufacturing process thereof, and particularly between inner and outer peripheries of the optical disk medium.
To record to the optical disk medium having such variance in the recording characteristics, with a conventional optical disk device, after the optimal recording power has been determined by the OPC, the optimal recording power is corrected with correction data that has been stored ahead of time (see Japanese Laid-Open Patent Application No. H11-328709, for example).
With the conventional information recording device, however, because of some kind of variance in the conditions, such as the recording characteristics of the optical disk medium, the recording speed, or various in the performance among individual devices in the OPC, even if recording is actually performed at the optimal recording power determined by the OPC, various problems may be encountered in terms of worse jitter or an increased reproduction error rate. This situation is not at all desirable from a standpoint of properly recording or reproducing data.
There are two types of factors that explain why the recording is not performed in the optimal state even if the recording is actually performed at the optimal recording power determined by the OPC. One is factors in the optical disk device that performs recording to the optical disk medium, and the other is factors in the optical disk medium itself.
In regards to the factors in the optical disk device, a difference between the recording power set for the optical disk device and the recording power of the light beam actually emitted at the recording power, slight variance in an oscillation frequency of a laser diode, an imbalance in a focus servo of the light beam, deviation between the light beam emitting lens face of an optical pickup in which a light source is installed and a face of the optical disk medium (this is called tilt), variance in recording pulse waveform of the light beam, and other such problems with manufacturing tolerance are possible factors preventing recording from being performed in the optimal state even when the recording is actually carried out at the optimal recording power determined by the OPC.
In regards to the factors in the optical disk medium, variance in recording sensitivity characteristics is the primary culprit. The variance in recording sensitivity characteristics mainly occurs in a process of manufacturing the optical disk medium. The optical disk medium has a recording layer and a protective layer produced by applying a coating of a polycarbonate or the like to a top face of the recording layer. To produce the optical disk medium efficiently, a step of applying the coating involves dropping a polycarbonate liquid near an inner periphery of a rotating optical disk medium. Then, the polycarbonate spreads to an outer periphery under the centrifugal force of the rotation.
In terms of the structure of the optical disk medium, in an ideal state the protective layer preferably has a uniform thickness over the entire surface of the optical disk medium. However, with the method discussed above, the thickness of the protective layer ends up being different at the inner and outer peripheries. Thus, the refractive index of the protective layer adversely affects reflectance of the light beam from the recording face at the inner and outer peripheries due to the difference in thickness, which tends to have an adverse effect on recording characteristics. The recording layer of the optical disk medium has lands and grooves. The lands form convex components. The grooves form concave components. The lands and grooves are formed in a spiral pattern in a track direction. The greater is the difference in reflectance between the lands and grooves, the better are the recording characteristics. Furthermore, the thickness and refractive index of the protective layer adversely affect the reflectance at the inner and outer peripheries as discussed above. In general, the thickness of the protective layer is less at the inner periphery, while the thickness of the protective layer is greater at the outer periphery. In addition to the thickness of the protective layer, if a coating agent contains a large amount of impurities, this can also increase the refractive index of the protective layer and have a major adverse effect on reflectance.
Also, the lands and grooves are formed by pressing a mold called a stamper against the optical disk medium under powerful pressure. However, when the lands and grooves are formed with the stamper that has been deformed by excessive use, the lands and grooves usually formed on a vertical or horizontal face with respect to the light beam are formed on a face that is inclined with respect to the light beam, which affect the reflectance. Similarly, reflectance is affected when warping occurs in the optical disk medium itself. Thus, with the optical disk medium, variance in the thickness of the protective layer, the impurity content of the protective layer, variance between stampers, and other such problems with tolerance in a manufacturing process are considered to be factors that may prevent recording from being performed in the optimal state even though the recording is actually performed at the optimal recording power determined by the OPC.
Because of the factors, even if the recording power is determined by the OPC in the test recording area, if any of the various types of variance mentioned above is present in the optical disk device, or if there is a large difference in the reflectance between the inner and outer peripheries of the optical disk medium, then recording cannot be performed in the optimal state. Thus, even if the recording power determined by the OPC is corrected, it will be difficult to accommodate a large change in the recording characteristics at the outer periphery, jitter will be worse, or the reproduction error rate will increase. As a result, video data cannot be outputted.
In view of the above, it will be apparent to those skilled in the art from this disclosure that there exists a need for an improved optical disk device. This invention addresses this need in the art as well as other needs, which will become apparent to those skilled in the art from this disclosure.